Circulating Liquid Cooling Systems
The vast majority of all positive displacement internal combustion engines presently operating throughout the world are cooled by pumping a water-based coolant in a closed circuit comprising cooling jackets around the combustion chambers and a heat exchanger (radiator). Some engines, mostly low-horsepower engines and some aircraft engines, are air-cooled, but air-cooling is poorly suited to large stationary and ground vehicle engines because it is impossible to maintain the reasonably stable temperatures that are required to ensure long engine life and good performance under various ambient conditions and loads.
Virtually all liquid-cooled engines use water or a solution of an antifreeze, such as ethylene glycol, in water. The use of water as a coolant has many advantages, such as its existence as a natural substance in plentiful supply in most parts of the world, lack of flammability, and excellent heat transfer characteristics. Its advantages far outweigh its disadvantages of causing corrosion and leaving deposits of impurities, both of which are largely overcome by additives in antifreezes in any case.
Over perhaps the last twenty years or so, and especially in recent years, there has been some increase in the operating temperatures of engine cooling systems, which is made possible by increasing the pressure of the system and using a higher temperature thermostat, in order to reduce the rate of heat rejection and improve the efficiency of the engine. Higher coolant temperatures improve efficiency not only by using more heat output in the thermal cycle rather than rejecting it but also by reducing quenching of the flame by keeping the combustion chamber walls hotter. On the other hand, higher temperatures and pressures in the cooling system cause maintenance problems, such as hose and coupling leaks and failures, and operating problems, such as a greater tendency to allow overheating of the engine, engine knocking, undesirably high oil temperatures and increased emissions of oxides of nitrogen (NO.sub.x).
Despite the recognized effectiveness of circulating liquid cooling there are also recognized shortcomings. It is necessary to provide a large volume of coolant and a heat exchanger large enough to handle the peak thermal load that the system will encounter. Otherwise, the engine will overheat from time to time and might be seriously damaged. These requirements add weight and expense to the system. The coolant is circulated from the top of the coolant jacket to the heat exchanger and returned to the lower part of the coolant jacket. This tends to create a fairly steep temperature gradient along the cylinder walls, which causes the cylinder diameter to vary from top to bottom. The rings have to expand and contract, which causes wear of the rings and ring lands. The lower portions of the cylinder walls are often at a temperature below the dew point of the water vapor present. Water vapor condensate mixed into engine lubrication oil will contaminate the oil and cause the formation of acids and sludge.
There are reports in the technical literature of early experiments with high temperature liquid coolants, such as ethylene glycol and aniline, used in pumped liquid systems (see Gibson, A. H., "Aero-Engine Efficiencies", Transactions of the Royal Aeronautical Society, No. 3, 1920; Frank, G. W., "High-Temperature Liquid Cooling", SAE Journal, Vol. 25, October, 1929, pp. 329-340; and Wood, H., "Liquid Cooled Aero Engines", SAE Journal, Vol. 39, July, 1936, pp. 267-287). Problems cited in these reports include instances of head temperatures running well above desired levels, distortion, hot spots, and leakage of coolant.
Young, infra, p. 635, discusses (writing in 1948) raising automotive engine coolant temperatures from, the then state-of-the-art, 60.degree. C. to 82.degree. C., to higher levels. He cautiously suggests that unpressurized ethylene glycol could be utilized as a coolant that would operate at a temperature higher than the boiling point of water but then observes (p. 635) that heat dissipation may decrease and "hot spots could also be expected in the average engine." Young concludes his discussion with suggestions of pressurized liquid systems using water-antifreeze solutions. The current state-of-the-art coincides with Young's concluding suggestions.
Bailey British Pat. No. 480,461 (1938) proposes a pressurized circulating water cooling system for aircraft engines supplemented by a condenser for collecting the steam generated under abnormally high loads, condensing the steam, and storing the condensate. A system of valves prevents return of the condensate until the engine is stopped and cooled down. The steam leaves the coolant jacket entrained within the pumped liquid flow and requires a "header tank" to separate the vapor from the liquid. As the egress of steam from the coolant jacket is dependent upon the rate of liquid coolant flow, a significant portion of the coolant jacket, particularly adjacent to combustion and exhaust areas, will become filled with vapor if the rate of vapor production becomes a substantial percentage of the rate of liquid coolant flow.
A gasoline-fueled automobile engine according to current technology utilizing a standard liquid cooling system, that pressurizes a coolant consisting of water and ethylene glycol in a 50/50 solution to a high pressure, say of the order of 172 KPa gauge (25 psig), and equipped with a thermostatic valve operating at 104.degree. C., appears to reach the upper limit of bulk coolant temperature that can be tolerated without unacceptable knock, thermal stress cracking of the engine head and other adverse effects of uneven and excessive engine temperatures. Indeed, unacceptable knock is frequently encountered after a few thousand kilometers of operation when carbon deposits that have built up on the combustion chamber domes begin to provide sites for glowing hot spots that cause preignition and detonation.
Ignition occurs in diesel engines when fuel is injected into a combustion chamber; thus preignition due to hot spots is not a problem as it is in gasoline spark-ignition engines. Nonetheless, uneven and excessive temperatures in a diesel engine, typical problems for an engine cooled by a conventional liquid cooling system, cause distortion and failure of components as well as increased engine emissions.